The invention relates to a display tube comprising a display screen and a neck accommodating an electron gun system for emitting an electron beam towards the display screen, and a deflection unit coaxially surrounding the display tube, which deflection unit comprises a line deflection coil system which, when energized, deflects the electron beam in a first direction and a field deflection coil system which, when energized, deflects the electron beam in a second direction transverse to the first direction, said deflection unit further comprising an annular correction coil which surrounds the path of the electron beam, is arranged transversely to the longitudinal axis of the tube and is connectable to a circuit providing a correction current for generating a correction magnetic field in the axial direction of the display tube.
The invention relates to both monochrome display tubes in which one electron beam is generated, and to colour display tubes in which three electron beams are generated.
In colour display tubes of the in-line type, the electron gun system is adapted to generate three coplanar electron beams which converge on the display screen. The deflection unit surrounding the display tube for deflecting the electron beams is used for deflecting the electron beams from their normal undeflected straight path into the one or the other direction so that the beams impinge upon selected pixels on the display screen on which they provide visual indications. By suitably varying the magnetic deflection fields, the electron beams can be moved up or down and to the left or the right across the (vertically arranged) display screen. By simultaneously varying the intensity of the beams, a visual presentation of information or a picture can be formed on the display screen. The deflection unit fixed around the cone section of the display tube comprises two deflection coil systems to enable the electron beams to be deflected in two directions which are transverse to each other. Each system comprises two coils arranged at opposite sides of the tube neck, with the systems being displaced about the tube neck by an angle of 90.degree. relative to each other. Upon energization, the two deflection coil systems produce orthogonal deflection fields.
The fields are essentially perpendicular to the path of the undeflected electron beams. A cylindrical core of magnetizable material which surrounds the line deflection coil system if it is of the saddle type, is generally used for concentrating the deflection fields and for increasing the flux density in the deflection area.
The deflection coils may be of the saddle type or (particularly the field deflection coils) of the type which is toroidally wound on the annular core.
After mounting a deflection unit provided with field deflection coils and line deflection coils on the display tube for which it is intended, twist errors sometimes appear to occur in operation. Briefly summarized, the problem is that the three electron beams are not located in a horizontal plane when they leave the gun but in a plane which is somewhat tilted (for example 1 degree maximum; tilted about the tube axis). The beams are directed towards the centre of the screen, i.e. the convergence is satisfactory in the screen centre; upon deflection, however, the tilt becomes manifest as (mainly) vertical blue-red errors across all further screen points, which errors are referred to as BRy errors. This problem becomes urgent when very stringent requirements are imposed on the convergence such as in CMT and HDTV applications.